As an antenna device employing MIMO (Multi-Input Multi-Output) technique for transmitting and receiving wireless signals of a plurality of channels simultaneously by using a plurality of antennas, a MIMO antenna device is disclosed in Patent Document 1, for example.
The conventional MIMO antenna device disclosed in Patent Document 1 includes four groups of antenna elements, the respective groups being arranged at even intervals, and a main body. Each group of antenna elements includes four antenna elements having polarization directions different from each other. Meanwhile, the main body includes a switch section connected to the antenna elements, a signal reception section receiving a reception signal via the switch section, an antenna control section generating a control signal for the switch section, an antenna selection section generating a combination of the antenna elements to inform the antenna control section of information of the selected elements, and an antenna determination section determining, based on the reception signal received by the antenna elements generated by the antenna selection section, a specific combination of the antenna elements to inform the antenna control section of information of the determined elements.
The conventional MIMO antenna device with the above-described configuration is intended to reduce correlation between antenna elements and ensure sufficient transmission capacity by determining a combination of the antenna elements in a manner that one antenna element is selected from each group of antenna elements.
That is, in the conventional MIMO antenna device, a plurality of antenna elements operate simultaneously and then each of the antenna elements obtains largest possible received power, thereby increasing total transmission rate of a plurality of signal sequences after MIMO demodulation. The MIMO antenna device described in Patent Document 1 achieves this by including more antenna elements in number than channels for concurrent communication and by selecting the antenna elements, each having larger received signal strength therefrom.
Such selection of the antenna elements is especially effective in mobile communication in the case where signal intensities of main polarization and cross polarization temporally vary or an arriving angle thereof varies, in accordance with a movement of a mobile station (user) and/or time-dependent change of an ambient environment. Further, a change in the polarization direction can be dealt with by using the antenna elements having different polarization characteristics from each other, and the time-dependent change can be overcome by controlling the antenna elements to be switched.
As described above, the MIMO antenna device described in Patent Document 1 including the plurality of groups of antenna elements, each group having the plurality of antenna elements, and can reduce correlation between the antenna elements or increase transmission capacity by causing the switch section to select a combination of the antenna elements having the weakest correlation therebetween or a combination of the antenna elements having the largest transmission capacity.
Further, with reference to Patent Documents 2 and 3, an example of a mobile wireless communication apparatus utilizing a portion thereof as an antenna will be described.
In a mobile wireless communication apparatus described in Patent Document 2, a part of a conductive housing of the mobile communication apparatus operates as a part of an antenna so as to aim at reduction of production costs, thinning and downsizing by reduction of the number of parts without employing dedicated parts for an antenna. Further, it is possible to configure a larger antenna by causing the housing itself to operate as the antenna, whereby higher sensitivity of the antenna can be expected. According to the mobile wireless communication apparatus described in Patent Document 2, high quality wireless communication can be expected, as to the portable telephone desired to be downsized, by causing the conductive housing to operate as a part of the antenna.
A mobile telephone described in Patent Document 3 is aimed at reduction of gain variation depending on a condition of a user's hand, and configuration of a lip-type mobile telephone 1 is disclosed, where a shield box 14 in the upper housing 3 and an output terminal of a transmission circuit 15 within the lower housing 4 are connected by a flexible cable 9, and the shield box 14 is used as an antenna (FIG. 3 of Patent Document 3). With such configuration where the shield box 14 is used as the antenna, the gain variation depending on the condition of the user's hand can be diminished.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2004-312381    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2004-274730    [Patent Document 3] Japanese Examined Patent Publication No. 3830773